Add a FixedOffset parser

[linkmauve]

In my project, an XMPP element parser, I need to be able to parse a timezone on its own, in order to implement XEP-0202.

I’m currently using a super ugly workaround by creating a DateTime from a random date concatenated with the timezone I want to parse, and then extracting the timezone from it.

A correct way to implement that could be to impl std::str::FromStr for FixedOffset, which would return only the timezone information and reject any other string.

[pitdicker]

Fixed in #1157.